1. Field of the Invention
The present invention pertains to a method of etching organic antireflection coatings (ARCs), and in particular, bottom antireflection coatings (BARCs). Organic antireflection coatings, as indicated by their name, include carbon and hydrogen-containing materials, and are typically polymeric in nature. The antireflection coatings are part of an etch stack which is used to produce semiconductor devices, and they are pattern etched to submicron dimensions. The present method permits control over shifts in the critical dimension of the etched feature size, while providing uniform etching of ARCs across the entire surface of a semiconductor substrate, despite variation in spacing between features across the substrate surface.
2. Brief Description of the Background Art
In the field of semiconductor device fabrication, it is well recognized that as device feature sizes decrease to about 0.18 xcexcm and smaller, mask patterning via photoresist materials requires the use of deep ultra violet wavelength (DUV) imaging radiation. Antireflective coatings are used in combination with DUV photoresists, among other photoresists, to reduce standing waves and back-scattered light, so that the dimensions of the patterning in the photoresist can be better controlled.
Generally, the photoresist is applied over a stack of other layers which are patterned as a part of the semiconductor device fabrication process. Some of the layers in the stack are consumed during the process of patterning underlying layers which become part of the functioning device. An ARC layer may be present at a number of different locations within a stack of layers, depending on the application. When the ARC layer is applied over the top of a layer stack, it is referred to as a top antireflective coating (TAR). When the ARC layer lies beneath the photoresist layer, it is commonly referred to as a bottom antireflective coating (BARC). TAR coatings are frequently removed during the photoresist patterning (developing) process, while BARC layers most often require a dry etching removal step.
Processes for the dry etching of organic ARCs usually are accomplished in a plasma etch system. ARC etching plasma source gases vary considerably in composition. Some examples of plasma source gas combinations include CHF3/CF4/Arxe2x80x94O2; CF4/Hexe2x80x94O2; O2/N2; HBr/O2; and HBr/CO2/O2xe2x80x94Ar.
In one process for etching organic antireflective coatings overlying a silicon-containing substrate, the substrate is placed into a process chamber and treated with a plasma. The plasma is generated from a process gas comprising oxygen and a compound selected from a group of compounds consisting of hydrogen and bromine-containing compounds, hydrogen and iodine-containing compounds, and mixtures thereof. Processing variables are adjusted to provide anisotropic etching of the organic antireflective coating.
In another etching process, an anti-reflection coating overlying a semiconductor substrate is etched by employing a plasma formed from a mixture of oxygen, nitrogen, and at least one inert gas. In an alternative method, the antireflective coating layer may be etched by employing a nitrogen plasma, which includes an inert gas, without any oxygen in the plasma, although the etch rate is said to be reduced.
Another method for plasma etching a BARC layer overlying a semiconductor substrate utilizes etch chemistry provided by a plasma processing gas which includes hydrogen bromide (HBr), CO2, and O2, with argon or another inert gas.
More information regarding the kinds of processes described above may be found in European Patent Publication No. EP 0820093, of Zhao et al., published Jan. 21, 1998; U.S. Pat. No. 5,910,453, to Gupta et al., issued Jun. 8, 1999; and European Patent Application No. EP 0859400, of Yang et al., published Aug. 19, 1998.
Uniformity of etching across a wafer has long been a concern, and, in general, the references pertaining to etching of organic antireflective coating layers place great emphasis on maintenance of the critical dimension of the feature being etched, such as a line width, contact pad dimension, gate size, and so on. Emphasis is also placed on the selectivity of the etch process, where the etch rate of an ARC layer is compared with the etch rate of an adjacent layer of material, such as a silicon-containing layer underlying the ARC layer, for example.
A very important variable, which has become more important with decreasing critical dimension size of etched features, is the etched feature critical dimension uniformity control across a substrate, such as a semiconductor wafer. For example, when the pattern being etched into an ARC layer is a series of lines and spaces, and the spacing between the lines is different at different positions on the substrate surface, the etch rate of the ARC may vary at different positions on the substrate surface. This may affect the depth of etch and may affect the profile of the feature being etched. It also affects the critical dimension uniformity across the substrate. The phenomenon of a change in overall etch performance across a substrate surface as a function of the spacing between etched features is sometimes referred to as a xe2x80x9cmicroloadingxe2x80x9d effect. Differences in etch rate and/or etched feature profile occur in part because the availability of etchant species at a given position on the substrate surface varies, and the amount of etch byproduct which is produced varies as well. One of the reasons that the availability of etchant species and byproduct residue varies across a wafer surface is that the input and distribution of processing gases and the removal of processing gases and etch byproducts from the processing chamber is frequently not uniform. Another reason is that there is different spacing between pattern features at different locations (positions) on the substrate surface. Etch byproducts tend to be generated by two different mechanisms: 1) the use of etchant gas compositions which contain significant amounts of passivating gases (e.g., N2, or polymer-generating gases, such as carbon-containing gases), and/or 2) back-sputtering of etched material onto feature sidewalls during etching. Etchant gas-generated passivation layers (1) tend to build up on feature sidewalls relatively evenly over the substrate surface. Back-sputtered passivation layers (2) tend to deposit more on isolated feature areas of the substrate, where there is more back-sputtering because the amount of material being etched tends to be greater than in dense feature areas of the substrate.
Commonly owned, copending U.S. application Ser. No. 09/611,085, of Shen et al., discloses a process for plasma etching of ARC layers which is said to provide critical dimension uniformity across a substrate surface. The ARC etching process utilizes CF4, HBr, and O2 chemistry. However, while this process provides excellent critical dimension uniformity across a substrate surface during etching of 1500 xc3x85 thick BARC layers, critical dimension uniformity is not as good when the process is used to etch thicker (e.g., 2000 xc3x85) BARC layers. This is believed to be due to the longer etch time needed to etch a thicker BARC layer. Therefore, it would be desirable to provide a process for etching a BARC layer that would provide excellent critical dimension uniformity across a substrate surface, when etching thicker ( greater than 1500 xc3x85 thickness) BARC layers.
The present invention includes a method of etching organic coating layers, and in particular, antireflective coating layers. The method provides improved critical dimension uniformity of the etched feature across the substrate surface, while providing selectivity favoring etching of the antireflective coating layer relative to an underlying silicon-containing layer. The present method has been shown to provide excellent critical dimension uniformity during etching of thicker ( greater than 1500 xc3x85) organic coating layers.
We have discovered a method for etching an organic coating layer which provides unexpected control over the etched feature critical dimension, as well as the critical dimension uniformity across a substrate surface (CD shift range), despite a difference in the spacing between etched features over the substrate surface.
A two-step method is used to etch an organic coating layer. The first step of the method is a main etch step, during which essentially the entire thickness of the organic coating layer is etched to endpoint. The main etch utilizes a plasma generated from a first source gas comprising a fluorocarbon gas and a non-carbon-containing, halogen-comprising gas.
The second step of the method is an overetch step, during which residual coating layer material remaining on feature surfaces following the main etch step is removed. The overetch step utilizes a plasma generated from a second source gas comprising a chlorine-comprising gas, an oxygen-comprising gas, and an inert gas (i.e., a noble gas). This chemistry provides excellent ( greater than 20:1) selectivity for etching the organic coating layer relative to an underlying silicon-containing layer. Power applied to bias the substrate during the overetch step is less than the power applied to bias the substrate in the main etch step.
With reference to a pattern of lines and spaces (by way of example and not limitation), the etch chemistry and process conditions used in the main etch step produces a smaller etched line width in dense areas (where features are spaced closer together) than the etched line width in isolated areas (where features are spaced further apart). Typically, the etched line width in both the dense areas and the isolated areas is smaller than the line width of the photoresist used to pattern the organic coating layer (such as an ARC). This reduction in line width from the target line width is referred to as a xe2x80x9cnegative CD shiftxe2x80x9d or a xe2x80x9cCD lossxe2x80x9d. Thus, the negative CD shift is greater in the dense areas than in the isolated areas during the main etch. The negative CD shift across the substrate is referred to as the xe2x80x9cCD shift rangexe2x80x9d.
As discussed above, the total amount of material being etched in isolated areas is greater than in dense areas; therefore, more polymeric material redeposits and builds up on isolated feature sidewalls during etching. This polymeric material build-up forms passivation layers which inhibit sidewall etching and slow the etch rate of isolated features. Further, the chemistry used in the main etch step includes CF4, which forms polymers which contribute to the build-up of passivation layers on isolated feature sidewalls during etching.
The etch chemistry and process conditions used in the overetch step produce a negative CD shift in the isolated areas which is greater than the negative CD shift in the dense areas. This is the opposite of the effect observed during the main etch step. Because most of the organic coating layer material has already been etched in the main etch step, build-up of passivation layers in isolated areas due to polymer deposition is no longer as much of a factor in the overetch step as it was during the main etch step. Further, the etch chemistry used in the overetch step does not include a polymer-producing (passivating) plasma source gas.
Another important processing variable is the substrate bias power used in the main etch step compared with the overetch step. The power applied to the substrate is lower in the overetch step than in the main etch step. As a result, there is less driving force to direct etchant species toward the substrate surface during the overetch step, which tends to slow the etch rate. This effect is more pronounced in dense feature areas than in isolated areas, because the narrower spacing between features in dense areas (higher aspect ratio) tends to protect exposed etched sidewall features from impacting reactive species. Therefore, the combination of etch chemistry and application of less substrate bias in the overetch step produces more rapid etching in isolated feature areas than in dense areas during the overetch step. Thus, there is a balancing effect when the main etch step is combined with the overetch step. This balancing effect reduces the CD shift range.
In summary, the present invention utilizes multiple etching steps which compensate for each other and take advantage of different mechanisms to achieve uniformity of etching across a substrate surface. Accordingly, disclosed herein is a method of etching an organic coating layer on a semiconductor substrate which utilizes a combination of the etch chemistries and substrate bias powers used in the main etch and overetch steps to balance etching in dense and isolated areas of a semiconductor substrate.